Rosetta STM is 'all shook up'

The test programme of the Rosetta Structural Thermal Model (STM)
continues to go with a bang. At the end of last week, the STM underwent
a series of shocking experiences in order to check its ability to survive
the rough treatment that will be meted out during launch. This was
followed today by a deployment test of a giant solar array.

The first of the STM's trials to be carried out last week was the so-called
'shogun' test. This was a joint operation by technicians from CNES,
Arianespace, and ESA's European Space Research and Technology Centre
(ESTEC) in The Netherlands. (Arianespace operates the Ariane 5 launch
vehicle that will send Rosetta on its way to Comet Wirtanen in 2003.)

The shogun test was a simulation of the shock transmitted to the
spacecraft when its protective fairing separates from the rocket's upper
stage. Using a copy of the actual spacecraft adapter which will attach
Rosetta to the upper stage, a set of pyrotechnic devices was attached
around the underside of the adapter. With all staff withdrawn to safe
positions behind protective glass, these explosive charges were set off
instantaneously to rupture the aluminium plate on the adapter.

"It only lasted a millisecond, but it went off with quite a loud crack,"
commented Rosetta Assembly and Integration Verification Engineer Alan
Moseley.

The next day, the STM was subjected to a 'clamp band separation test'.
The clamp band is a metal strip that attaches the launch adapter to the
spacecraft. After checking that the clamp band fitted properly, technicians
once again had to discover whether its explosive separation would damage
Rosetta. This time, two pyrotechnic charges were set off. As the band was
split in two, both sections were pushed away from the spacecraft interface
by springs and caught by special 'catcher' brackets.

"Although the tests themselves were extremely short, the shocks imparted
to the STM were quite severe. The detailed analysis of the impact on the
spacecraft's units will data take several days, to come to a realistic
conclusion," said J. C. Salvignol, Rosetta mechanical systems engineer.

"I'm pleased to say that the STM passed both tests without any damage,"
added his colleague, Jacques Candé.

Today saw the deployment test for one of Rosetta's 16 metre-long solar
arrays. These enormous arrays are needed because Rosetta will be operating
at five times the Earth's distance from the Sun, where levels of sunlight are
only 4% those on our planet. Rosetta will be making history as the first
spacecraft ever to use solar arrays to generate electrical power during a
deep space mission beyond the asteroid belt.

The check-out began when six 'thermal knives' were used to melt through
the attachments which held the array to the side of the spacecraft. Using
a special jig, springs on the giant panel caused it to slowly open out to its
full, impressive length. After 3 minutes 47 seconds, the array was fully
extended, allowing engineers to check its alignment and condition.

Later in the day, yet another shock test took place. This time, the dish-
shaped high-gain antenna was shaken by three pyrotechnic devices which
exploded one after another. During a subsequent performance check of the
antenna motor, the dish was successfully moved more than 40 degrees
from its fixed position.

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ESA's Rosetta comet chaser unveiled in London

The final design of the European Space Agency's Rosetta comet chaser
was revealed today at the Royal Society in London when a 1:4 scale (7.1m
diameter) model of the giant spacecraft was unveiled by ESA's Science
Director, Professor Roger Bonnet.

The ceremonial unveiling was also witnessed by representatives of the
Particle Physics and Astronomy Research Council (PPARC), which funds
the UK contribution to the Rosetta mission, members of the media, and
more than 100 scientists and engineers from around the world.

"The expertise of European scientists in cometary exploration has been
absolutely outstanding, from the early days of astronomy with Halley
and Newton, through to Ludwig Biermann who discovered and explored
the solar system and its interaction with comet tails." said Prof. Bonnet.
"Then Giotto flew to Halley's Comet 13 years ago. Now we have Rosetta,
which builds on this experience."

The Rosetta Spacecraft

The mission involves two spacecraft -- a 3 tonne orbiter, which will
fly alongside Comet Wirtanen's icy nucleus, and a small lander, which
will touch down on the nucleus itself. The Rosetta orbiter will carry
12 scientific experiments, with a further nine on the lander.

A 1:4 scale model was chosen for today's unveiling because of the
impossibility of finding a venue large enough to contain the full-size
version. The flight model of the Rosetta Orbiter spacecraft is 32 metres
across, so large that it would stretch the entire width of a football
pitch. Since Rosetta will have to operate more than 720 million km
(450 million miles) from the Sun, where light levels are only 4% of
those on the Earth, it has to carry giant solar panels to provide
electrical power in the dark depths of the Solar System.

One of the main themes of the event was the importance of Rosetta
as a Cornerstone mission in ESA's Horizons 2000 science programme.
"Rosetta is a flagship mission for European solar system exploration.
It is one of four missions that will land on a foreign body in the first
10 years of the next century. ESA spacecraft will lead the way with
landings on Mercury, Saturn's moon Titan, and even on a comet", said
Prof. Bonnet.

Speakers also explained that, apart from its scientific importance,
Rosetta is also a significant source of technological innovations which
can be used for other missions such as Mars Express. This enables ESA
to make efficient use of resources and reduce development costs for
other science projects.

"Rosetta will be a tremendous technological challenge," said Project
Manager Bruno Gardini. "We have less than four years to build the
largest, most sophisticated spacecraft ever to visit a comet."

"There are many challenges ahead," he added. "Rosetta will have to
survive a hazardous eight year trek across 5.25 billion km of space.
It will then have to rendezvous with a comet which is travelling
towards the Sun at over 130,000 km per hour. After releasing a lander
onto its tiny nucleus, it will have to fly alongside the comet as it
swoops towards the Sun."

The Rosetta Mission

Although ESA's Giotto spacecraft flew past two comets in 1986 and
1990, many questions remain unanswered. Rosetta is designed to
unravel the mysteries surrounding these primitive objects.

"Space exploration is all about discovering the unknown," commented
Rosetta Project Scientist, Dr. Gerhard Schwehm. "Just as, 200 years
ago, the discovery of the Rosetta Stone eventually enabled Champollion
to unravel the mysteries of ancient Egyptian hieroglyphics, so Rosetta
will help scientists to unravel the mysteries of comets."

Dr. Schwehm went on to explain that comets are among the oldest,
and least altered, objects in the Solar System. They are thought to
have existed, almost unchanged, for the last 4.5 billion years, and are
regarded as the building blocks from which the planets formed.

Comets are also important sources of information for scientists
studying how our planet evolved and life began. One theory suggests
that a comet collision wiped out the dinosaurs 65 million years ago.
Such impacts were much more frequent when the Earth was young. It
seems that comets arrived in such vast numbers that they may have
delivered a significant fraction of the water in our oceans. Furthermore,
some scientists believe that the organic (carbon-based) molecules found
in comets were the 'seeds' from which life evolved on Earth 4 billion
years ago.

Dr. Alan Fitzsimmons of Queen's University, Belfast, commented,
"Through PPARC, the UK is funding the Open University's MODULUS
instrument on board Rosetta and also supporting the Plasma Science
Package. This will allow European scientists to decipher the physics
and chemistry of comets in unprecedented detail, and ensures that
the UK plays a key role in this exciting mission."

Rosetta and the British Museum

ESA's comet chasing spacecraft is named after the Rosetta Stone,
one of the most famous exhibits at the British Museum. Starting on
10 July, the Rosetta Stone will be the centrepiece of the Museum's
exciting new 'Cracking Codes' exhibition. Hundreds of thousands of
visitors from around the world are expected to flock to this exhibition
during the next six months. Models of the Rosetta Orbiter and Lander
will be on display throughout the event.

"The Rosetta Stone has been described as the most famous piece
of rock in the world, and is one of the most visited objects in the
British Museum," said Dr. Richard Parkinson of the Museum's
Department of Egyptian Antiquities. "This month marks the 200th
anniversary of its discovery by Napoleon's troops at Rosetta in Egypt.
The Museum is celebrating the event with a revolutionary redisplay of
the newly conserved stone, and a special exhibition, entitled `Cracking
Codes', to show the full impact of the Stone on our understanding of
the past."

"The use of the stone for decipherment of Ancient Egyptian was not a
single event but a continuing process of scientific investigation. This
is why we are particularly delighted to be able to include a model of
the Rosetta space probe to show how decipherment is very much to do
with the future and not just with the past," he added.

The Return of Giotto

Today's press briefing coincided with the London meeting of the Rosetta
Science Working Team and the second Earth flyby of ESA's remarkable
Giotto comet probe. Giotto's brief homecoming took place almost 14
years to the day since its launch on 2 July 1985 and five years after
its previous return to Earth's vicinity on 2 July 1990.

Despite a peppering from dust particles travelling faster than bullets,
Giotto survived its encounter with Comet Halley to return the first
detailed, close-up pictures of a comet nucleus. Six years later, the
remarkably robust spacecraft made history once more when it visited
a second comet.

During today's flyby, the now deactivated spacecraft swept to within
220,000 km of the planet (just over half the Earth-Moon distance).
Travelling at a speed of about 3.5 km/s, Giotto's trajectory took it
over the South Pole and southern South America before it headed once
more into deep space.

"Giotto paved the way for Rosetta," said Gerhard Schwehm. "It was the
Agency's first planetary mission and was a tremendous success, both
technically and scientifically. It provided a wealth of scientific results
and gave scientists the unique chance to study two different comets
with the same set of instruments."

Rosetta Lander unveiled

A full size model of the Rosetta lander was presented to the public for
the first time today. The unveiling ceremony took place at the Institute
for Space Simulation in Cologne-Porz, a facility operated by the German
Aerospace Research Centre (Deutschen Zentrums für Luft- und Raumfahrt
or DLR).

Dignitaries present at the press conference included the German
Parliamentary State Secretary, Wolf-Michael Catenhusen, and DLR
Chairman, Professor Walter Kröll.

Also available to answer questions from the press were ESA Project
Scientist Gerhard Schwehm, DLR lander project manager Stephan Ulamec,
and lander lead scientist Helmut Rosenbauer from the Max-Planck-Institut
für Aeronomie.

"The Rosetta lander will play a key role in our investigation of Comet
Wirtanen," said Dr. Schwehm. "It will provide ground truth about the
nucleus which will complement the orbiter's measurements of the
comet's composition, surface and subsurface conditions."

The box-shaped Rosetta lander measures approximately 1 metre across
and 80 cm in height. The flight version will be attached to the side of
the Rosetta orbiter during the journey to Comet Wirtanen, then soft land
on the surface of the icy nucleus. Data from the lander will be relayed
back to Earth via the orbiter.

The structural and thermal model of the lander presented at the meeting
will be used for vibration and thermal tests. These are needed to ensure
that the probe will survive the hazards of shaking during launch and
extreme temperature variations during its 9-year-long voyage to the
comet.

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ESA's Rosetta mission and the puzzles that Hale-Bopp left behind

While Comet Hale-Bopp steams away into the outer darkness,
not to return to the Sun's vicinity for many centuries, the
European Space Agency and multinational teams of space
scientists are finalizing plans to examine another comet at very
close quarters, in the Rosetta mission.

The scientific payload was confirmed by ESA's Science
Programme Committee in February. Now the scientists must
perfect the full range of ultra-sensitive yet spaceworthy
instruments in good time for Rosetta's despatch by an Ariane 5
launcher in January 2003. And even as most of the world was
admiring Comet Hale-Bopp at its brightest, dedicated
astronomers were examining the comet that will be Rosetta's
target.

Although too faint to be seen with the naked eye, Comet
Wirtanen made its closest approach to the Sun on 14 March
and a fairly close approach to the Earth on 24 March. This
comet comes back every 5.5 years. Rosetta will dance
attendance on Comet Wirtanen, not at the next return in 2002,
nor even in 2008, but in 2013. The project is an ambitious and
patient effort to achieve the most thorough investigation of a
comet ever attempted.

As the successor to ESA's highly successful Giotto mission to
Halley's Comet and Comet Grigg-Skjellerup (which took
seven years) Rosetta will spend eight years positioning itself. It
will manoeuvre around the planets until it is shadowing Comet
Wirtanen far beyond Mars, on nearly the same path around the
Sun. In 2011 it will rendezvous with the comet and fly near it.
In April 2012 Rosetta will go into a near orbit around Comet
Wirtanen, and escort it for 17 busy months, as it flies in to
make its closest approach to the Sun in September 2013, at the
climax of the mission.

"The Giotto mission placed us at the forefront of cometary
exploration," comments Roger Bonnet, ESA's director of
science. "The motivation came from European scientists with a
sharp sense of the special importance of comets for
understanding the Solar System. The same enthusiasm drives
us onward to Rosetta, which will ensure our continued
leadership in this important branch of space science."

Scientific tasks

During its prolonged operations in very close company with
the comet's nucleus, Rosetta will map and examine its entire
surface from distances of 10 to 50 kilometres with a set of
remote-sensing instruments. As the spacecraft moves around
the nucleus at a very leisurely walking pace, other onboard
instruments will analyse the dust and vapours, which will
emanate from Comet Wirtanen with ever-increasing vigour as
the Sun's rays warm it.

Rosetta will drop a lander on to the comet's surface, for close
inspection of its physical condition and chemical composition.
The lander is a venture led by Germany, France and Italy, with
participation from Austria, Finland, Hungary, Poland and the
UK. As a box packed with scientific instruments and standing
on three legs, the lander will be capable of anchoring itself to
one spot and drilling into the surface.

It may also be able to hop like a flea to visit another part
of the nucleus. A combination of solar energy and electric
batteries will enable operations to last for several months.

"The combination of Rosetta in orbit around the comet and the
lander on its surface is very powerful from a scientific point of
view," says Gerhard Schwehm, ESA's project scientist for
Rosetta. "We shall watch Comet Wirtanen brewing up like a
volcano as it feels the heat of the Sun. In place of hazy
impressions of the nucleus of a comet half hidden by its dust
clouds, we shall see all the details with unprecedented clarity."

Unanswered questions

During and after the 1986 appearance of Halley's Comet,
comet science made great progress. More recent comets have
revealed important secrets to ESA's Infrared Space
Observatory and to other space telescopes examining them at
wavelengths unobservable from the Earth. Yet basic
questions about comets remain unanswered.

Just as the Rosetta Stone was the key that unlocked the
meaning of Egyptian hieroglyphs, so the Rosetta spacecraft is
intended to decipher the meaning of comets and their role in
the origin and history of the Solar System. Here are a few of
the main puzzles.

What does a comet weigh? Guesses about the density of
cometary material vary widely, and only an orbiting spacecraft
can give exact measurements of the comet's volume and mass.

Is a comet a dirty snowball or an icy dirtball? In other words,
is it made of ices contaminated with mineral and tarry dust, or
is it a consolidation of dust coated with ices?

Why is the nucleus of a comet so dark? Giotto established
that Halley's nucleus is like brownish-black velvet, absorbing
96 per cent of the sunlight falling on it. Is the colour due to a
surface deposit of tarry dust, or is the interior dark too?

Why are small regions of a comet highly active when most of
its surface is not? Multiple jets of dust seen emanating from
Halley's Comet, and spectacularly from Comet Hale-Bopp,
imply that certain hot-spots differ physically or chemically
from the rest of the comet's surface.

Is a comet made as single piece, or does it consist of loosely
joined blocks, as suggested by the Giotto images? This relates
to the questions of how comets are built, and why they break
up into smaller fragments, as seen spectacularly with Comet
Shoemaker-Levy 9 which hit Jupiter in 1994.

Does a dying comet evaporate and disappear, or does it
simply exhaust the stocks of ice that drive the emissions of gas
and dust from an active comet? If the latter answer is correct,
dead comets persist long afterwards as dark, inactive masses
of minerals and tar, and pose a lasting threat of collisions with
the Earth.

What is a comet's exact composition? Many ingredients are
known, and the approximate abundances of the main
constituents. Details coming from Rosetta will pin down (1)
how comets were fashioned from similar constituents of
interstellar dust and (2) how comets contributed to building
the planets, including the Earth, and stocking their
atmospheres.

Is the tarry, carbon-rich material in comets a jumble of every
kind of chemical that inorganic processes can make from
carbon, nitrogen, oxygen and hydrogen, or does it contain
special compounds? This is relevant to assessing the role of
comets in the origin of life on the Earth.

The comet specialist Uwe Keller of the Max-Planck Institut fur
Aeronomie, Germany, is one of the Giotto veterans who has
helped with the planning of Rosetta. He was in charge of
Giotto's camera.

"Rosetta is the mission we are all waiting for," Dr Keller
comments. "After I spent six years analysing our images of the
Halley nucleus, I say that basic scientific assumptions about
the nature of comets are still contradictory. We shall settle the
arguments only by the close, prolonged inspection that Rosetta
will make possible."

Engineering the Rosetta mission

To build up the speed needed to adopt the same orbit around
the Sun as Comet Wirtanen, Rosetta must steal energy of
motion from the planets, in a swingby of Mars and two
swingbys of the Earth. During its far-flung manoeuvres in
pursuit of the comet, Rosetta will inspect the asteroids
Mimistrobell and Rodari at close quarters. When Rosetta is far
from the Earth, or on the wrong side of the Sun,
communication will be difficult.

The spacecraft will therefore have a high degree of robotic
self-reliance. It will also be capable of hibernating for more
than two years without attention -- a technique devised by
ESA for the later stages of the Giotto mission.

Rosetta will rely on solar power, even when more than five
times further than the Earth from the Sun. Special
low-intensity solar cells are under development for Rosetta.
Conditions in this farthest phase of Rosetta's voyage will be
very chilly, but ESA's engineers are satisfied that the
temperatures inside the spacecraft can be kept within limits by
black paint, multilayer insulation and electric heaters. Despite
its originality and sophistication, Rosetta will be just a flying
box with solar arrays like wings, looking rather like a
telecommunications satellite.

"Keep it simple," is the motto of John Credland, ESA's project
manager for Rosetta. "Simplicity brings reliability," he
explains, "and that is my overriding concern for the
engineering of a spacecraft that has to survive and operate far
from the Earth for nearly eleven years."

To command Rosetta, and to receive its signals carrying new
of the comet, ESA will use a new 32-metre deep-space
tracking antenna at Perth in Australia, and a 15-metre antenna
in Spain. The spacecraft operations, especially in the
near-comet phase of the mission, will be a novel experience for
the controllers at the European Space Operations Centre in
Darmstadt, Germany.

The gravity of the comet will be weak, and Rosetta's
manoeuvres around it will be like a ballet in slow motion. At
around 10 kilometres distance, the spacecraft will travel at
only 1-2 kilometres per hour in relation to the comet and take
about a week to circle once around the nucleus. Sometimes
Rosetta will swoop even closer to the comet's surface, to
inspect possible landing sights and to drop the lander. The
spacecraft's thrusters will adjust the orbit. To keep
manoeuvres to a minimum, and so conserve fuel and avoid
polluting the comet's environment, computer simulations will
help the spacecraft navigators to predict the consequences of
any manoeuvre for weeks in advance.

The target comet

Present-day space propulsion systems allow a rendezvous only
with a comet with a predictable and relatively small orbit
around the Sun. All comets of this kind are "old", in the sense
that they have visited the Sun's vicinity many times and are no
longer vigorous in the dust and gas formation that makes their
visible comas and tails. The second comet visited by Giotto,
Comet Grigg-Skjellerup, was of this elderly kind.

From among several short-period candidates, the mission team
chose Comet Wirtanen as Rosetta's target comet because it
offered the quickest timetable between the launch of the
spacecraft and the completion of the mission.

The comet was discovered by chance by Carl Wirtanen in
1948 on photographic plates at the Lick Observatory in
California. In 1972 and 1984 encounters with the planet
Jupiter reduced the size of Comet Wirtanen's orbit, and
shortened the interval between its visits to the Sun from 6.65
to 5.5 years.

Despite many observations no one really knows the comet's
mass, size and shape. The uncertainties are reflected in the
computer simulations of manoeuvres near the comet. These
cover a wide range of possibilities from a lightweight comet to
a massive one, and from a small comet 1 kilometre in diameter
to a large one 20 kilometres wide. The best estimate may be
1.5 kilometres. But it is in the nature of a voyage of
exploration like Rosetta's that you don't know what you will
find!